2,152 research outputs found
Non-metric chaotic inflation
We consider inflation within the context of what is arguably the simplest
non-metric extension of Einstein gravity. There non-metricity is described by a
single graviscalar field with a non-minimal kinetic coupling to the inflaton
field , parameterized by a single parameter . We discuss the
implications of non-metricity for chaotic inflation and find that it
significantly alters the inflaton dynamics for field values , dramatically changing the qualitative behaviour in this regime.
For potentials with a positive slope non-metricity imposes an upper bound on
the possible number of e-folds. For chaotic inflation with a monomial
potential, the spectral index and the tensor-to-scalar ratio receive small
corrections dependent on the non-metricity parameter. We also argue that
significant post-inflationary non-metricity may be generated.Comment: 7 pages, 1 figur
Bouncing Palatini cosmologies and their perturbations
Nonsingular cosmologies are investigated in the framework of f(R) gravity
within the first order formalism. General conditions for bounces in isotropic
and homogeneous cosmology are presented. It is shown that only a quadratic
curvature correction is needed to predict a bounce in a flat or to describe
cyclic evolution in a curved dust-filled universe. Formalism for perturbations
in these models is set up. In the simplest cases, the perturbations diverge at
the turnover. Conditions to obtain smooth evolution are derived.Comment: 7 pages, 1 figure. v2: added references
Creep of a fracture line in paper peeling
The slow motion of a crack line is studied via an experiment in which sheets
of paper are split into two halves in a ``peel-in-nip'' (PIN) geometry under a
constant load, in creep. The velocity-force relation is exponential. The
dynamics of the fracture line exhibits intermittency, or avalanches, which are
studied using acoustic emission. The energy statistics is a power-law, with the
exponent . Both the waiting times between subsequent
events and the displacement of the fracture line imply complicated stick-slip
dynamics. We discuss the correspondence to tensile PIN tests and other similar
experiments on in-plane fracture and the theory of creep for elastic manifolds
The post-Newtonian limit in C-theories of gravitation
C-theory provides a unified framework to study metric, metric-affine and more
general theories of gravity. In the vacuum weak-field limit of these theories,
the parameterized post-Newtonian (PPN) parameters and can
differ from their general relativistic values. However, there are several
classes of models featuring long-distance modifications of gravity but
nevertheless passing the Solar system tests. Here it is shown how to compute
the PPN parameters in C-theories and also in nonminimally coupled curvature
theories, correcting previous results in the literature for the latter.Comment: 5 pages, no figures; To appear in PRD as a rapid communicatio
Anisotropic fluid inside a relativistic star
An anisotropic fluid with variable energy density and negative pressure is
proposed, both outside and inside stars. The gravitational field is constant
everywhere in free space (if we neglect the local contributions) and its value
is of the order of , in accordance with MOND model. With
, the acceleration is also constant inside stars but the
value is different from one star to another and depends on their mass and
radius . In spite of the fact that the spacetime is of Rindler type and
curved even far from a local mass, the active gravitational energy on the
horizon is , as for the flat Rindler space, excepting the negative sign.Comment: 9 pages, refs added, new chapter added, no figure
Unifying Einstein and Palatini gravities
We consider a novel class of gravity theories where the connection is
related to the conformally scaled metric with
a scaling that depends on the scalar curvature only. We call them
C-theories and show that the Einstein and Palatini gravities can be obtained as
special limits. In addition, C-theories include completely new physically
distinct gravity theories even when . With nonlinear ,
C-theories interpolate and extrapolate the Einstein and Palatini cases and may
avoid some of their conceptual and observational problems. We further show that
C-theories have a scalar-tensor formulation, which in some special cases
reduces to simple Brans-Dicke-type gravity. If matter fields couple to the
connection, the conservation laws in C-theories are modified. The stability of
perturbations about flat space is determined by a simple condition on the
lagrangian.Comment: 17 pages, no figure
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